Synthesis and Electronic Characterization of Iridium‐Aluminum and Rhodium‐Aluminum Heterobimetallic Complexes Bridged by 3‐Oxypyridine and 4‐Oxypyridine

Abstract

We demonstrate the synthesis and characterization of regioisomeric late transition metal‐aluminum heterobimetallic complexes containing 3‐ and 4‐oxypyridine ligands which bridge between the aluminum and transition metal center (Rh or Ir). Combined experimental and theoretical data allow for direct comparison of the electronic effect of the two different aluminum‐containing ligands on the late transition metal center. In combination with electronic data regarding previously reported 2‐oxypyridine bridged systems, we report a measurable decrease in ligand donor power as the regiosubstitution of the oxypyridine bridge is varied from 2‐ to 4‐ to 3‐. In all cases, addition of aluminum is found have a minimal effect on the donor ability of the oxypyridine bridge in dichloromethane solution.

Systematic evaluation of the electronic effect of aluminum-containing ligands in iridium–aluminum and rhodium–aluminum bimetallic complexes

“…The aluminum-containing heterobimetallic complexes of iridium (18a-21a) and rhodium (18b-21b) were synthesized by adding triisobutylaluminum to the appropriate precursor, similar to our previous approach (Scheme 4, for full details, see the Experimental Section). 44 The 1 H NMR spectra of complexes 18a-21a and 18b-22b contain diagnostic proton signals between δ −0.29 ppm and δ 0.02 ppm similar to those observed in our previous reports. [42][43][44] Observed second-order doublets of doublets correspond to the diastereotopic methylene protons α to the aluminum center.…”

“…The observed trend in donor power among regioisomers was explainable via a simple resonance analysis of the organic bridge. 44 In all cases, addition of the nitrate-bound dialkylaluminum moiety into the transition metal-oxypyridine skeleton resulted in a mild increase in the electron donor strength, relative to a proton, of the bridging ligand. 44 Observation of electronic trends in our oxypyridine-bridged complexes constitutes the first-step toward a thorough understanding of how changes in the structure of an aluminum-based ligand affects the electronic properties of the heterobimetallic complex.…”

“…Synthesis of our new suite of heterobimetallic complexes was undertaken using the "transition metal first" strategy commonly exploited in our laboratory. 42,44 Vaska's complex, [Ir(PPh 3 ) 2 (CO)Cl] (1a) or its rhodium congener, [Rh(PPh 3 ) 2 (CO)Cl] (1b, NOTE: all complexes denoted "a" contain Ir and complexes denoted "b" contain Rh), were first reacted with a silver salt in the presence of the desired pyridinemethanol (Scheme 1), hydroxypyridine (Scheme 2), or hydroxyquinoline bridging ligand (Scheme 3) resulting in complexes 8a-17b. Monometallic precursors were then reacted with triisobutylaluminum to generate the desired bimetallics 18a-21b (Scheme 4).…”

“…15,24,27,28 Recently, we published a trio of studies investigating the electronic structure and reactivity of a series of iridium-aluminum and rhodium-aluminum complexes wherein the metals were bridged by regioisomers of oxypyridine (Figure 1). [42][43][44] Our most recent study highlighted the correlation between regiosubstitution of the oxypyridine bridge and the resultant donor power of the aluminum-containing ligand to the late transition metal. The observed trend in donor power among regioisomers was explainable via a simple resonance analysis of the organic bridge.…”

“…Electronic data obtained from this study are tabulated and compared with that from our previous reports. 42,44 Finally, trends relating the structural features of the aluminum-containing ligands and their donor ability are discussed. Gratifyingly, our hypothesis derived from our previous publications, that the donor strength of aluminum-containing ligands correlates to the donor strength of the bridge between the two metal centers, is supported by the expanded library.…”